The present invention relates to an ultrasound horn of rotation-symmetric type, comprising a supply end which is connected to an ultrasound source, and a tubular working end which displays an endless work surface.
Ultrasound vibrations within the frequency range of 15-50 kHz are nowadays employed industrially for a number of purposes, inter alia welding of different material types. Welding of thermoplastic material is a common task within the packaging industry and, as a result, ultrasound welding finds an increasing number of fields of application in the manufacture of different types of packaging containers, not only of pure plastic film or plastic material but also of different types of laminates which include exterior layers of thermoplastic. Originally, the ultrasound technique was employed only for relatively simple, rectilinear welds of planar material, but continued progress and development have entailed that ultrasound welding is now also employed for advanced welding operations, e.g. different combinations of materials and different types of non-linear welds in a plurality of dimensions.
A typical assembly for ultrasound sealing of the type which is employed within the packaging industry utilizes an ultrasound source (drive unit or converter) for creating ultrasound at the desired frequency. The ultrasound source may be of conventional type and include, e.g. a piezoelectric crystal which is caused to oscillate by being connected to a suitable current source. Once the ultrasound source has thus brought about the conversion from electricity to mechanical, reciprocating motion, this oscillation is normally transferred by mechanical contact between the ultrasound source and an ultrasound horn at whose opposite end is located the actual work surface which is in contact with the material which is to be welded and sealed. The work surface of the ultrasound horn (or in any event its operative part) is normally more or less linear and may be, for example, straight, curved or circular. A feature common to the majority of prior art applications of ultrasound welding of thermoplastic packaging materials is that the work surface is located at a right angle to a centre line extending through the ultrasound source and the ultrasound horn. The transfer of the ultrasound vibrations from the ultrasound source to the ultrasound horn hereby takes place rectilinearly along the centre axis of the horn, which reduces losses and creates relatively few problems in connection with the design of the horn and its work surface, since only axial waves are transferred through the assembly. In prior art assemblies, there is also often provided a so-called booster located between the ultrasound source and the horn, the boosterxe2x80x94because of its geometric designxe2x80x94amplifying the amplitude of the mechanical motion so that this is optimal for ultrasound welding of, for example, thermoplastic material.
In the manufacture of, for example, round, polygonal or conical packaging containers, it is desirable to realize a closed sealing line which extends around the circumference of the packaging container. This is, for example, necessary when a round packaging container is to be provided with end walls, or when the circumferential casing of a round packaging container is to be connected to a more or less conical container top. A closed seal along, for example, the circumference of a cylindrical packaging container may naturally be realized by means of an ultrasound horn with a short, straight work surface, provided that the sealing operation takes place in many stages during simultaneous rotation of the packaging container. However, this technique is time-consuming and not always realizes a satisfactory result, for which reason it is desirable to realize an ultrasound horn with an endless, for example, circular or hexagonal work surface. When round or conical packaging containers are to be provided with end walls or more or less conical tops, use is therefore preferably made of an ultrasound horn which has a circular work surface and thereby realizes a closed, annular weld in one single working phase. One example of this is shown in U.S. Pat. No. 3,438,824 which discloses the ultrasound sealing of an end wall to a container. In such instance, use is made of a horn which is rotation-symmetrical or annular apart from the anchorage portion, the horn being supplied radially from a conventional ultrasound source. This design and construction permit welding of surfaces which are substantially parallel with the longitudinal axis of the welded object, but provide no amplitude amplification, and the unit is relatively bulky and inflexible. For example, it is necessary to displace each individual packaging container which is to be welded to a work position in which the packaging container is in contact with the horn. This is a disadvantage in high output capacity machines where the packaging containers pass on a conveyor or a mandrel wheel and the sealing equipment must be moved between a retracted, inactive position and a protracted, active working position.
A construction displaying axial supply and thereby consequential possibilities of displacing the ultrasound assembly reciprocally between working and rest positions is disclosed in U.S. Pat. No. 4,063,990, which describes the use of an ultrasound horn for the fixed welding of opening arrangements onto packaging containers. Those surfaces which are to be welded together are, in this instance, located at a 90xc2x0 angle to the longitudinal axis of the horn, which gives optimum conditions for the transfer and utilisation of the axial vibrations. If those surfaces which are to be welded together are instead located more or less parallel with the centre axis (in any event at an angle of less than 45xc2x0 to the centre axis), the welding by means of axial vibrations will be ineffective, since, with the reducing angle, an increasingly large proportion of radial vibrations is required in order to achieve a satisfactory and practically usable welding effect.
It will thus be apparent from the foregoing that, particularly within the packaging industry, there is a need for an ultrasound horn which, despite axial supply, gives radial vibrations or oscillations of such amplitude that welding can take place along a circular or polygonal work surface which is located at a relatively slight angle to the centre axis of the horn.
There is thus a need in the art to realise an ultrasound horn which, without causing disruptive parasite oscillations and thereby losses in the form of heat and material stresses, converts axially supplied ultrasound oscillations into oscillations with a radial component which has sufficiently large amplitude to make for ultrasound welding in cylindrical or conical packaging container surfaces.
One object of the present invention is to realise an ultrasound horn of rotation-symmetrical type, the horn converting incoming, axial ultrasound oscillations into ultrasound oscillations with a radial component of such amplitude that ultrasound sealing becomes possible along an endless, circular or polygonal work surface whose angle to the centre axis of the ultrasound horn is equal to or less than 45xc2x0.
A further object of the present invention is to realise an ultrasound horn of the above-outlined type whose work surface is located such that axially projecting parts of the processed object do not come into contact with the horn.
Still a further object of the present invention is to realise an ultrasound horn of the above-outlined type which gives the desired conversion of axial oscillations into radial oscillations without the losses and thereby heating of the horn disrupting the work cycle.
Yet a further object of the present invention is to realise an ultrasound horn of the above-outlined type which displays a simple and reliable design and construction making for lengthy, continuous operation without the creation of harmful oscillations or crack formation.
Still a further object of the present invention is to realise an ultrasound horn which does not suffer from the drawbacks and weaknesses inherent in the above-mentioned prior are assemblies and which, therefore, may be employed for commercial operation in modern, high output capacity filling machines.
The above and other objects have been attained according to the present invention in that an ultrasound horn of the type disclosed by way of introduction has been given the characterizing features that the mean diameter of the tubular part varies along the axis of the horn, and that the tubular part has, at its end portion provided with the work surface, a first region of greater mean diameter and cross sectional area than a second region which is located at the end portion of the tubular part facing towards the supply end.
Preferred embodiments of the ultrasound horn according to the present invention have further been given the characterizing features as set forth in the appended subclaims.
By designing a rotation-symmetrical ultrasound horn according to the present invention with a tubular part which has greater mean diameter and cross sectional area at the work end than at the supply end, a conversion of axial oscillations into radial oscillations takes place along the periphery of the tubular part, this conversion being further amplified by a suitable selection of wall thickness and be selecting a mean circumference which, in the region at the work surface, is substantially equal to one wavelength.